Although applicable to any inductor component, the present invention will be described in combination with inductive components with a high fill factor.
In modern electric and electronic devices winding arrangements for inductive components are an important component. Inductors are especially used in power conversion devices like buck converters and boost converters.
In order to reduce the size of such power conversion devices the working frequencies of said devices become higher. For small power converters up to 10V the working frequencies have risen into the MHz range. For middle sized power converters up to 200V and high power converters up to 500V the target frequency is about 300 kHz to 1 MHz.
In such power conversion devices the inductive components (inductors or transformers) are an important factor regarding losses and size. Particularly, the size of the inductive components should be as small as possible, the shape should be square and the AC/DC resistance ratio should be as low as possible at the desired working frequency.
Common inductive elements—like shown in FIG. 16 comprise a toroidal core TC with a litz or strand wire SW wound around the core TC. Inductors like the one shown in FIG. 16 have a favorable AC/DC current ratio, but such conductors are relatively big and the fill factor is small, especially when additional isolation is required in order to implement secondary windings in transformer applications. Furthermore, the shape of such inductive components is inconvenient to use in modern power conversion devices.
With the constant increase of the working frequency of such power conversion devices the so called “skin effect” becomes more and more relevant when designing power conversion devices. The skin effect is responsible for the current being conducted in a skin area of the conductor, wherein the skin depth δ becomes smaller with higher frequencies. The skin depth δ is about 0.1 mm or less for frequencies in the MHz area. Therefore, the thickness of the conductors of such common inductive elements like the one shown in FIG. 13 is limited to 0.2 mm (2δ). Consequently, the increase of the working frequency results in thinner conductors. The thinner the conductors with round intersection are, the higher the number of litz wires in the litz or strand wire needs to be to conduct the load current. A high number of litz wires results in an even worse fill factor of such inductors.
Inductors can also comprise flat band conductors instead of litz wires. Such inductors are shown in FIGS. 13 and 14, respectively.
FIG. 13 shows an inductor with a magnetic core 1″″, wherein the magnetic core 1″″ has two winding windows 2a″″ and 2b″″. FIG. 13 also shows the flux lines that build up in such an inductor.
A certain percentage of flux lines inevitably passes the winding windows 2a″″ and 2b″″, which effects that not all of the winding turns N1, N2 include the same flux causing differences in induced voltage in individual turns. Specifically, as seen in FIG. 13, the core flux Φ surrounds the winding windows 2a″″ and 2b″″, while the stressed flux line Φ″ passes the winding windows 2a″″ and 2b″″. The turn N1 includes Φ1 flux lines, while the turn N2 includes Φ2 flux lines. The flux Φ1 includes complete core flux Φ′ and a part of stressed flux Φ″ that is represented by Φ1″, while the flux Φ2 includes the complete core flux Φ′ and a part of the stressed flux Φ″ that is represented by Φ1″ and Φ2″. Since the stressed flux Φ2 is greater than the stressed flux Φ1, and the changes of flux over time are increased as more flux lines are included and the induced voltage in the turn N2 is greater than in turn N1.
In the case of all the winding turns N1, N2 being connected in series, as it is commonly used for the windings of inductive components, the difference in the induced voltage of the winding turns in different positions in the winding windows 2a″″ and 2b″″ has no negative effect, because the induced voltages of all winding turns N1, N2 are summed up and therefore cause no equalizing currents.
In order to reduce the ohmic losses caused due to high frequency current, the demand for thinning the conductor thickness increases drastically. The thickness thinning of the conductors with round intersection results in increase of the number of litzes in the strand in order to be able to conduct the load current. The thinner the litz wires are the worse the fill-factor of such winding is. Thinning the square intersection flat conductors limits the maximum possible load current. The load current can be increased by the expansion of the winding window, which is possible only to certain limits set due to the outside inductor dimension ratio. Division of the individual flat conductor strips into more strips is not possible, since interleaving, which is normally used in litz strand conductors cannot be achieved.
However, the flat wires do achieve a much better fill factor than litz wires, since they present an advantage in the possibility of compensating the thinning of the conductors by increasing the width of individual conductors. The simultaneous increase of the length of the winding windows 2a″″ and 2b″″ is possible only within certain limits, therefore in such multi-layer windings single flat band conductors connected in parallel to form a single winding presents a possible solution.
Despite the equalizing currents in litz or strand wires being negligible the fill factor deteriorates the high frequency operation for high currents applications, since with the frequency increase the isolator/conductor ratio raises.
Besides the voltage change occurring due to the different position of the winding turns N1, N2 in the winding windows 2a″″ and 2b″″ there are also other aspects that deteriorate the high frequency operation for high current applications. The load current of individual winding turns N1, N2 influences the current in all of the other turns of the same winding by creating its own magnetic field causing longitudinal circular current flowing on the inner and outer side of the individual conductor with respect to the core. These longitudinal circular currents are summed up with the load current, such that the load current is increased on the inner side of the conductor and decreased on the outer side of the conductor, this phenomena is called proximity effect. The consequence of the proximity effect are greater ohmic losses with the increase of frequency.
Using flat band conductors in parallel solves the skin and proximity effect, while simultaneously allowing the same load current to flow through the winding as the effective conductive area remains the same. Specifically, FIG. 14 shows a magnetic core 1′″ with a winding with a single conductor which is divided into two parallel flat band strips S1″ and S2″ isolated between each other and surrounding the gap GW″. The parallel flat band strips S1″ and S2″ are short circuited in connection areas 3 providing taps T1 and T2 to form a single conductor is demonstrated in FIG. 14.
Dividing individual conductors into flat band strips solves the fill factor, skin effect and proximity effect issue at the same time. The flux leakage into the area of the winding windows 2a″″ and 2b″″ cannot be removed. The flux tends to flow through low permeability areas such as isolator or air in the winding window area and partly through the conductors. The gap GW″ between both parallel conductor strips S1″ and S2″ presents an area for the flux lines ΦW to penetrate into it resulting in a voltage difference ΔV among individual parallel conductor strips S1″ and S2″ of the same conductor.
Therefore, an additional voltage causing longitudinal current IWL through parallel conductor strips S1″ and S2″ and both connection taps T1″″, T2″″ appears, as demonstrated in FIG. 15. In FIG. 15 a winding W″ is shown, with two parallel conductor strips S1″ and S2″ and the gap GW″ between the parallel conductor strips S1″ and S2″, wherein the flux ΦG penetrates the gap GW″. This voltage equalizing longitudinal current IWL is added to the load current as the summation of both contributions. The induced longitudinal current IWL is a problem in paralleled conductor strips which is similar to the problems caused by the proximity effect.
Document WO 2007/136288A1 shows a method for winding a high-frequency transformer by winding a strip of electrically conductive material around a core in two parallel windings.